Coated steel sheet and method for manufacturing the same

ABSTRACT

A coated steel sheet having a coated layer on surfaces of a steel sheet of a composition containing not less than 0.1 mass % and under 3 mass % of Al, wherein a following condition
     A or B is met:   A: An AlN precipitate layer exists on a matrix side near an interface between said steel sheet and said coated layer   B: Oxide of Al exists in said matrix right under said surfaces of said steel sheet.

TECHNICAL FIELD

The present invention relates to a coated steel sheet and a method formanufacturing the coated steel sheet suitable for technical fields, suchas automobiles, construction materials, and household appliances.

BACKGROUND ART

In recent years, in fields, such as automobiles, construction materials,household appliances, and so on, the use of high-tensile steel sheet hasincreased. Particularly in the automotive industry, the adoption ofhigh-tensile steel sheet is increasing rapidly with a view to reducingcar weight for better fuel economy and improved crashworthiness.

In addition to high tensile strength, to provide ductility to withstanda complicated press-forming, development has been actively pushedforward for a high-ductile high-tensile steel of a structure containingretained austenite to utilize a strain-induced transformation of thisretained austenite.

As an example of high-tensile steel sheets, there is one that has acomposition including addition elements such as Si, Mn, Ti, Al and P,which is disclosed in Japanese Patent Publication No. 3-51778. It iswell known, however, that as the Si content is increased, an Si oxidefilm is formed on the surfaces of the steel sheet during an annealingprocess, thus deteriorating the steel properties, such as chemicalproperties, electro-galvanized coating adhesion, hot-dipgalvanizability, and hot-dip galvanized coating adhesion. Above allelse, the big problem is the inferior hot-dip galvanizability ofSi-containing high-tensile steel sheets, i.e., the applied zinc does notadhere to some portions of the steel sheet (so-called “bare-spot”) inhot-dip galvanizing, or the adhesion of the coating is insufficient.When a steel sheet contains not less than 0.1% carbon by mass, there hasbeen difficulty in carrying out galvanizing or forming a stablegalvanized layer even on a continuous hot-dip galvanizing line, whichincludes a RTH (all Radiant Tube Heating) type furnace or a NOF (NonOxidizing Furnace) type furnace.

As a method for increasing ductility and tensile strength withoutincreasing the Si content, a technique for achieving high ductility andhigh tensile strength has been disclosed, in which instead of increasingthe Si content, the Al content in the steel is positively increased tothereby prevent the surface quality deterioration peculiar to theSi-added steel and simultaneously make the retained austenite stable(JP-A-5-171344).

However, because Al and Si are readily oxidizable elements, in additionto the Si oxide film, an Al oxide film is formed during annealing, andas with the Si-added steel, it has been impossible to preventdeteriorations in the hot-dip galvanizability and the galvanized coatingadhesion in the Al-added steel sheet.

It is generally well known that Al is an element to deteriorateweldability. To make an Al-added steel practically applicable, it hasbecome an imperative requirement to improve its spot-weldability.

When a high-tensile steel sheet is used for automobiles, after chemicaltreatment or electrodeposition coating, a top coat is applied whennecessary, and as demand is mounting for rust resistance in recentyears, the improvement of corrosion resistance after anelectrodeposition process is increasingly important. However, ahigh-tensile steel sheet, which contains a large amount of galvannealingelements with high reactivity, is poorer in corrosion resistance thanmild steel. For this reason, if one tries to further improve corrosionresistance, there is a problem of difficulty in increasing highstrength.

The present invention has been made to solve the above problems in theprior art, and has as its object to provide a coated steel sheetsuperior in coating adhesion even if the base sheet is an Al-containingsteel sheet, and also provide a method for manufacturing this coatedsteel sheet.

DISCLOSURE OF THE INVENTION

To solve the above problems, according to the present invention, thereis provided a coated steel sheet which has a coated layer on a surfaceof a steel sheet of a composition containing not less than 0.1 mass %and under 3 mass % of Al, wherein there is an AlN precipitate layer onthe matrix side near the interface between the steel sheet and thecoated layer or there is an oxide of Al in the steel matrix right underthe surface of the steel sheet.

Further, the coated layer preferably is a hot-dip galvanized layer andcontains 0.1˜1.0 mass % of Al.

Further, the coated layer preferably is a Zn—Fe galvanneal coating thatfurther contains 7˜15 mass % of Fe.

Further, the AlN precipitate layer preferably has a thickness of notless than 1 μm and not more than 100 μm.

In addition, the steel composition preferably further contains one ortwo kinds selected from not less than 0.1 mass % of Si and not less than0.5 mass % of Mn.

Moreover, the steel composition preferably further contains one or twokinds selected from not less than 0.01 mass % and not more than 1 mass %of Mo and not less than 0.005 mass % and not more than 0.2 mass % of Nb.

Furthermore, the steel composition preferably further contains not lessthan 0.01 mass % and not more than 0.5 mass % of Cu, not less than 0.01mass % and not more than 1 mass % of Ni, and not less than 0.01 mass %and not more than 1 mass % of Mo.

Moreover, the steel composition preferably further contains 0.03˜0.25mass % of C, 0.001˜1.0 mass % of Si, 0.5˜3.0 mass % of Mn, and0.001˜0.10 mass % of P.

Further in the steel matrix, there are preferably one or more kindsselected from oxides of SiO₂, MnO, FeSiO₃, Fe₂SiO₄, MnSiO₃, Mn₂SiO₄, andP₂O₅.

Further, the amount of oxides in total per one side surface ispreferably 0.01˜1.0 g/m².

Further, the steel composition preferably contains 0.01˜1.0 mass % of Moand 0.005˜0.2 mass % of Nb.

Further, the steel composition is preferably 0.01˜0.5 mass % of Cu,0.01˜1.0 mass % of Ni, and 0.01˜1.0 mass % of Mo.

Moreover, the coated layer is preferably galvannealed.

In addition, the Fe content in the galvannealed coated layer ispreferably 7˜15% by mass.

On the other hand, a method for manufacturing a coated steel sheetaccording to the present invention comprises the steps of heating andholding a steel slab, and hot-rolling the slab and hot-dip-galvanizing ahot-rolled steel sheet, wherein the slab contains not less than 0.1 mass% under 3 mass % of Al, and the above-mentioned holding is carried outin an atmosphere containing not less than 1 vol % and not more than 20vol % of O₂ and not less than 70 vol % of N₂ under the conditions thatmeet an equation (1) shown below and the above-mentioned galvanizing isperformed by using a galvanizing bath with an Al concentration in thebath is 0.14˜0.24 mass % at a bath temperature of 440˜500° C.{Heating and holding temp. (° C.)−(1050+25Al)}×heating and holding time(min)≧3000  (1)wherein Al denotes an Al content (mass %) in the steel.

Preferably, the steel sheet is galvanized by using a galvanizing bath ofAl concentration of 0.10˜0.20 mass % in the bath at a bath temperatureof 440˜500° C. and the hot-dip-galvanized layer is further subjected toa galvannealing process at 460˜550° C.

Further, preferably, cold-rolling is carried out between the hot-rollingprocess and the hot-dip galvanizing process.

Moreover, the steel slab preferably further contains one or two kindsselected from not less than 0.1 mass % of Si and not less than 0.5 mass% of Mn.

Additionally, the slab preferably further contains one or two kindsselected from not less than 0.01 mass % and not more than 1 mass % of Moand not less than 0.005 mass % and not more than 0.2 mass % of Nb.

Moreover, the slab preferably further contains not less than 0.01 mass %and not more than 0.5 mass % of Cu and not less than 0.01 mass % and notmore than 1 mass % of Ni, and not less than 0.01 mass % and not morethan 1 mass % of Mo.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a picture showing an electron microscope observation result ofan AlN precipitate layer;

FIG. 2 is a picture showing AlN precipitate layer analyzed by EPMA; and

FIGS. 3A to 3D are graphs showing effects of holding temperature andholding time on the coating adhesion and the occurrence of the AlNprecipitate layer when the slab was heated.

BEST MODE FOR CARRYING OUT THE INVENTION

Description will be made of a coated steel sheet, and particularly of ahot-dip galvanized steel sheet and a method for manufacturing thehot-dip galvanized steel sheet.

(1) FIRST EMBODIMENT

Description will start with a chemical composition of a steel sheet as abase sheet for coating as a first embodiment of the present invention.Note that in the following description, the contents of elements insteel are mentioned only by percents, but they should all be construedas figures in percents by mass.

(Not Less than 0.1% Under 3% of Al)

In this first embodiment, steel sheets that contain not less than 0.1%of Al are used. The reason is as follows. So long as the Al content insteel is under 0.1%, a decrease in the coating adhesion caused by asurface segregation of Al is less of a problem, or AlN is not formed,which will be described later. Further, in this embodiment, from aviewpoint of securing a strength-ductility balance, it is preferable toprovide a steel structure that contains retained austenite; however, ifthe Al content is under 0.1%, the retained austenite is unstable, sothat the steel is required to contain not less than 0.1% of Al, which isadequate from a point of view of attaining abetter strength-ductilitybalance. Note, however, that with steel sheets of Al content of not lessthan 3.0 mass %, as described later, even if AlN is formed in thesurface layer of the steel sheet, the amount of Al surface segregationduring annealing increases, and it is difficult to obtain an improvedquality of the coating adhesion even though one does what one can do informulating a better method of generating a nitride layer. Consequently,the Al content is limited to under 3.0%.

(One or Two Kinds Out of not Less than 0.1% of Si and not Less than 0.5%of Mn)

Si and Mn may be contained since they are conducive to high strength.Above all else, not less than 0.1% of Si and not less than 0.5% of Mnare preferably contained from a viewpoint of achieving high strength.However, Si content of over 1.0% and Mn content of over 0.5% makes itdifficult to secure weldability and coating adhesion; therefore,preferably, the upper limit of the Si content is limited to 1.0% and theupper limit of Mn to 3.5%.

(One or Two Kinds Out of not Less than 0.01% and not More than 1% of Moand not Less than 0.005% and not More than 0.2% of Nb)

Mo and Nb reduce the grain size of the matrix structure and retard therecrystallization, to thereby promote the internal oxidation of Al inthe temperature rising process. In this way, Mo and Nb have an effect ofsuppressing the surface segregation of Al. Therefore, Mo and Nb can becontained in steel to obtain a better coating adhesion. This effectappears at a Mo content of not less than 0.01% and the Nb content of notless than 0.005%. However, when the Mo content is over 1%, inhot-rolling in the production process of a base sheet for a galvanizedsteel sheet, there appears a tendency for the surface property todeteriorate. Further, when the Nb content is over 0.2%, there is atendency that the steel hardness rises and the rolling propertydeteriorates. Therefore, preferably, Mo is contained in a range of notless than 0.01% and not more than 1%, and Nb in a range of not less than0.005% and not more than 0.2%. Mo and Nb maybe added in the rangesmentioned above. (Not less than 0.01% and not more than 0.5% of Cu, notless than 0.01% and not more than 1% of Ni, not less than 0.01% and notmore than 1% of Mo)

When Cu, Ni and Mo are added, the coating adhesion is improved. Theimprovement mechanism of the coating adhesion by addition of Cu, Ni andMo has not been clarified, but it has been confirmed that when thoseelements are added together, but not separately, the internal oxidationof Al is promoted to thereby suppress the surface segregation of Al, andthe coating adhesion is improved.

As for other components, considering production cost and workability ofan automotive steel sheet, it is suitable to use a steel sheet whichcontains 0.0005˜0.25 mass % of C, 0.001˜0.20 mass % of P, and0.0001˜0.01 mass % of S. Besides those elements, to further control thestrength-ductility balance, it is no problem if a steel sheet is usedwhich contains not more than 0.15 mass % of Ti, not more than 1 mass %of Cr, and 0.001˜0.005 mass % of B to control the balance betweenstrength and ductility of steel sheet. The rest preferably consists ofFe and unavoidable impurities.

Description will now be made of the AlN layer formed in the surfacelayer, the AlN layer serving as an important factor of this embodiment.

In this embodiment, an AlN precipitate layer where Al exists chiefly asAlN is formed in the surface layer of a steel sheet. For this reason, inthe heating step before the galvanizing process, the Al in steel in thesurface layer is fixed as nitride in the matrix and is inhibited fromdiffusing to the surface region of the steel sheet.

It has been recognized that when an AlN precipitate layer exists, thereis an effect that Si and Mn, which are readily oxidizable elements likeAl, are inhibited from increasing their densities in the surface layerwhen the steel is annealed, though the reason is unknown. Therefore,even with steel sheets which contain relatively large amounts of Si andMn and therefore the coating adhesion is liable to deteriorate or“bare-spot” areas tend to occur, in the presence of an AlN precipitatelayer, favorable hot-dip galvanizability and better coating adhesion canbe obtained.

What is called the AlN precipitate layer here is a layer in which 20% ormore of Al in the basic steel exists as nitride. When the amount of Alpresent as nitride is under 20%, the Al present in a solid solutionphase is over 80%, and the Al existing in solid solution phase diffusesinto the surface of the steel sheet, and therefore the above-mentionedbettering effects of coating adhesion is reduced.

The amount of Al that precipitated out as AlN is obtained by a method asfollows. For a predetermined thickness (in steps of 5 μm, for example)from the surface layer, a predetermined amount of the AlN precipitate isdissolved by an electrolytic method using 10 w/v % of acetylacetone-1w/v % of tetramethyl ammonium chloride-methanol, and insoluble residuesare analyzed, by which the presence of AlN can be conformed. Theresidual AlN is decomposed by steam distillation, and by separating anddetermining only the quantity of N, the quantity of the N in the AlNprecipitate can be determined. On the basis of this value, the quantityof Al in the AlN precipitate can be determined. Further, the quantity ofa solid-solution Al can be determined by evaporating the rest other thanthe residue to dryness, again dissolving the residue in acid, andcalculating the quantity of Al by an atomic absorption method compliantwith JIS G 1257. From the above results, the proportion of nitride of Alin the AlN precipitate layer can be calculated.

Note that because the presence of AlN can be confirmed by EPMA analysisof the sectional area of a steel sheet and by analyzing both N and Al,the presence of AlN can be conformed by EPMA analysis in a simplemanner.

FIG. 1 is a picture showing an SEM electron microscope observationresult of a sectional area of a steel sheet where an AlN precipitatelayer is formed on the surface layer. FIG. 2 is a picture showing ananalysis result of the presence of Al by EPMA. According to FIGS. 1 and2, Al exists as nitride in a columnar or angular form and is distributedin a region 10˜20 μm depth from the interface of the steel matrix, andit is obvious that in that portion of the region where AlN did notprecipitate out, the quantity of solid solution of Al is not much. Thisregion corresponds to the AlN precipitate layer. Therefore, the solidsolution of Al is inhibited from diffusing from this region to thesurface during annealing, and therefore the coating adhesion does notdeteriorate. It is understood that in an area much deeper than thatregion, the presence of nitride is not recognized, but Al is presentmostly in a solid solution phase.

The thickness of the AlN precipitate layer is preferably not less than 1μm and not more than 100 μm. The reason is as follows. If there is someamount of AlN precipitate layer in the surface layer, an effect thatprevents surface segregation of Al will appear, and this effect becomesconspicuous when the thickness of the AlN precipitate layer is 1 μm ormore. It is practically difficult to form an AlN precipitate layer witha thickness of over 100 μm, and if the thickness is over 100 μm, theinfluence on the quality of material becomes not negligible.

By hot-dip galvanizing a base sheet for a hot-dip galvanized steel sheetwhich has the steel composition and the AlN precipitate layer asdescribed above, a hot-dip galvanized steel sheet with excellent coatinglayer adhesion can be obtained.

Description will be made of a method for manufacturing a hot-dipgalvanized steel sheet according to this embodiment.

As with an ordinary steel sheet for hot-dip galvanizing, a steel sheet(a base sheet) for hot-dip galvanizing according to this embodiment ismanufactured by heating and holding a steel slab made by continuouscasting or the like, subjecting the slab to a hot-rolling process orfurther to a cold-rolling process. In the present invention, in order toprevent Al from segregating at the surface in the annealing processbefore hot-dip galvanizing, it is necessary to have an AlN precipitatelayer previously formed on the surface layer of the steel sheet beforeor after annealing.

With regard to a method for forming an AlN precipitate layer, theinventors hit upon a conception that by nitriding the Al in the steelsurface layer when the steel slab was heated and held, it would bepossible to make Al in the surface layer exist as AlN in the subsequenthot-rolling, acid-cleaning and cold-rolling processes. On the basis ofthe above conception, the present inventors investigated the conditionsfor heating and holding a steel slab.

Al is well known as an element that can be easily nitrided. As a resultof our study, it has become clear that Al is nitrided more in preferenceover other elements when it is heated at a high temperature in anatmosphere chiefly consisting of N₂ and including O₂. The mechanism bywhich O₂ promotes the nitriding of Al has not necessarily beenclarified. In an O₂-bearing atmosphere, oxides are formed in largeamounts on the surface of the steel and the oxides serve as a diffusionpath of nitrogen, a fact which is considered to be one factor of O₂assisting in nitriding Al. It has also been clarified that in order togenerate nitride of Al while the slab is being heated and held, the O₂concentration is required to be at least 1 vol % or higher. Raising theO₂ concentration to 20 vol % or higher requires some other method forblowing oxygen into a heating and holding furnace and also acceleratesoxidation of the steel matrix itself; therefore, it is necessary tolimit the O₂ concentration to not more than 20 vol %. It does not matterif CO and CO₂, other than the components O₂ and N₂, are mixed, but N₂needs to be 70 vol % or higher to generate nitride of Al.

In an N₂-bearing atmosphere, by heating and holding the above-mentionedslab at a high heating and holding temperature and in a prolongedheating and holding time, the Al in the surface layer of the slab can benitrided. At this time, if the amount of Al in the steel is large, theheating and holding time to nitride the Al becomes longer accordingly.In this respect, on steels with varying Al contents, inquiry was madeinto the effects of the heating and holding time and temperature beforehot-rolling on the adhesion of the hot-dip galvanized coating.

In practice, a slab of a composition containing 0.1˜3% of Al, 0.5% of Siand 2.2% of Mn was used, in an atmosphere of 3 vol % of O₂ and the restconsisting of N₂, the slab was hot-rolled to a thickness of 2.8 mm. Theoxide scale formed on the surface of an obtained hot-rolled steel sheetwas removed by acid cleaning, and then the steel sheet was cold-rolledto a thickness of 1.6 mm, annealed at 810˜825° C., and subjected toaveraging at 400˜460° C. Subsequently, the steel sheet was hot-dipgalvanized in a hot-dip Zn bath with an Al concentration of 0.13 mass %and continuously treated by a galvannealing process at 500° C.

Samples for evaluating the coating adhesion were taken from a Zn—Fegalvanneal coated steel sheet. The coating adhesion was evaluated asfollows. A test specimen to which a cellophane tape was attached wasbent 90 degrees on the tape-attached side and bent back, the tape waspeeled off, and the peeled-off amount of the coating per unit length wasmeasured in terms of counts of Zn by a fluorescent X-ray method. Samplesranked first or second were classified as good (∘, Δ) and those rankedthird and so on were classified as defective (X) by referring to thecriteria in Table 1.

TABLE 1 X-Ray Fluores. Counts Rank   0~under 500 1  500~under 1000 21000~under 2000 3 2000~under 3000 4 3000 or more 5

The evaluation results are shown in FIGS. 3A to 3D. According to FIGS.3A to 3D, before hot rolling, a slab is heated and held under theconditions that the heating and holding temperature, the heating andholding time and the Al content in the steel meet equation (1), by whicharrangement a hot-dip galvanized steel sheet with excellent coatinglayer adhesion can be manufactured.{Heating and holding temperature (° C.)−(1050+25Al)}×heating and holdingtime (mm)≧3000  (1)where Al denotes an Al content (mass %) in the steel.

As a result of observation of a cold-rolled steel sheet to check thepresence or absence of an AlN precipitate layer, it was confirmed thatan AlN precipitate layer was formed in the surface layer when the aboveequation (1) was satisfied.

When a steel sheet with a chemical composition that contains not lessthan 0.1% and under 3% of Al is subjected to heating and holding beforehot rolling in an atmosphere with an O₂ content of not less than 1 vol %and not more than 20 vol % under the condition that meets the equation(1), a steel sheet with an AlN precipitate layer in the surface layercan be manufactured. Moreover, though containing Al and other readilyoxidizable elements such as Si and Mn, the steel sheet has an excellentcoating adhesion after it has been hot-dip galvanized.

Note that the AlN formed by the above-mentioned method is not onlyincluded in the steel of the surface layer but also sometimes existsexposed on the surface of the steel matrix. Even in such a case, the AlNhave no effects on the ductility and mechanical properties of the steelsheet nor has it effects on the surface quality such as coating layeradhesion. It is considered because the AlN precipitate layer is confinedto the uppermost of the surface layer and its exposure at the surface ofthe steel matrix is limited to a very few parts thereof.

The hot-rolled steel sheet is obtained by hot rolling after heating andholding under the above conditions, and the steel sheet is thensubjected to acid cleaning or acid cleaning plus cold rolling andannealing, and furthermore the steel sheet is hot-dip galvanized.

The acid cleaning of the hot-rolled steel sheet is done to remove theoxide scale formed on its surfaces. The conditions for acid cleaning arenot limited to specific items, but because the AlN precipitate layer isrequired to remain, consideration must be given to prevent the steelmatrix from being dissolved in large amounts during acid cleaning. Theacid is preferably hydrochloric acid, but other acids such as sulfuricacid may be used. The concentration of the acid is preferably 1˜20 mass%. To prevent the steel matrix from dissolving in large amounts, aninhibitor (dissolution inhibitor) is preferably added.

Cold rolling may be performed if necessary to control the mechanicalproperties and the thickness of a final product. When cold rolling iscarried out, the draft is preferably 30% or higher to promoterecrystallization in subsequent annealing. However, if the draft is 80%or higher, large load is applied to the rolling mill, making rollingdifficult; therefore, the draft is preferably 30˜80%.

Annealing just before hot-dip galvanizing may be by a method in whichhot-dip galvanizing is performed successively after annealing has beendone by well-known so-called continuous annealing, or by a method inwhich a steel sheet annealed once (primary annealing) is cooled, andthen the steel sheet is acid-cleaned to activate its surface and alsoafter the surface oxide formed by primary annealing is removed by acidcleaning, the steel sheet is heated again (secondary annealing) andsuccessively hot-dip galvanized. However, the annealing process justbefore galvanizing preferably includes a step for uniformly heating thesteel sheet at least partially under a reducing atmospheric conditionchiefly consisting of H₂—N₂ with a view to deoxidizing the Fe-basedsurface oxide film and securing the hot-dip galvanized coatingwettability. Or it is no problem if a process may be employed in whichan Fe-based oxide film is formed in the temperature-rising process in anNOF (Nonoxidation furnace) type heating furnace, for example, and theoxide film is deoxidized. Note that primary annealing is preferably at750˜930° C. to obtain an appropriate structure. If the primary annealingtemperature is higher than 930° C., the readily oxidizable elements suchas Si increases its segregation at the surface which adversely affectsthe galvanizability and the galvannealability. The secondary annealingis preferably at 650° C. or higher with a view to deoxidizing the oxidefilm formed during acid cleaning, and preferably at not less than 850°C. with a view to preventing the coarse-grain, for example, of the steelstructure.

Acid cleaning after primary annealing is performed, for example, by amethod of light acid-cleaning at 60° C. for several seconds in ahydrochloric acid of about 5 mass % or so. Other acids such as sulfuricacid may be used. Generally, acid cleaning is preferably performed withacidity of pH≦1, temperature of 40˜90° C. for 1˜20 sec. If thetemperature is below 40° C. and the time is less than 1 sec, the removaleffect of surface segregates cannot be obtained, and if the temperatureis higher than 90° C. and the time is longer than 20 sec, excessive acidcleaning occurs, which results in roughening of the surface.

A steel sheet is provided with a strength-ductility balance as follows.When annealing and hot-dip galvanizing are performed continuously in acontinuous annealing process, it is desirable that after intercriticalheating is finished, while bainite transformation is made to take placeby averaging at 350˜500° C. for not less than two minutes to letsegregates of C appear in the austenite, and continuously performinghot-dip galvanizing. When hot-dip galvanizing is performed after primaryannealing, cooling, acid cleaning, and secondary annealing, preferably,after intercritical heating by primary annealing, the steel sheet isquenched below 300° C. at a rate of not less than 40° C./s to obtain ahardened structure consisting of a ferrite-martensite phase, andimmediately before galvanizing, a tempering process is carried out byheating to 725˜840° C., and then cooling at a rate of not less than 5°C./s to form a composite structure of ferrite-temperedmartensite-retained austenite, and finally hot-dip galvanizing isperformed.

As a method for forming the AlN precipitate layer, description has beenmade of a method for adjusting the heating and holding conditions beforehot rolling. Incidentally, a steel sheet for hot-dip galvanizingaccording to the present invention can be manufactured by annealing thesteel sheet in an atmosphere of nitriding elements, such as an H₂—N₂system gas, mixed with trace amounts of CO and NH₃ in the annealingprocess just before hot-dip galvanizing.

Description will be made of a hot-dip galvanized steel sheet accordingto this embodiment.

A hot-dip galvanized steel sheet according to this embodiment can beobtained by hot-dip galvanizing a steel sheet for hot-dip galvanizingpurpose, which includes the above-mentioned AlN precipitate layer. TheAlN layer remains on the matrix side near the interface between thesteel sheet and the hot-dip galvanized layer after the hot-dipgalvanizing process. The hot-dip galvanized steel sheet obtained asdescribed has a good coating adhesion because the readily oxidizableelements such as Al, Si and Mn are inhibited from segregating at theinterface between the steel matrix and the coating layer.

The hot-dip galvanized layer (hereafter referred to simply as thecoating layer) is preferably a coating layer of a composition containing0.1˜1% of Al or an a Zn—Fe galvanneal coating layer that furthercontains 7˜15% of Fe in addition to the above-mentioned chemicalcomposition.

With a hot-dip galvanized steel sheet without galvannealing the coatinglayer (this steel sheet is hereafter referred to as GI), if the Alcontent in the coat layer is below 0.1%, the Fe—Zn galvannealingreaction shows rapid progress during the galvanizing process; therefore,unevenness occurs in the external appearance. With the GI, above allelse, the Al content is preferably not less than 0.2% to inhibit thegalvannealing reaction. When the Al content in the coating layer is over1%, in the galvanizing process, the Fe—Al alloy layer becomes thickwhich is formed on the coating layer side close to the interface betweenthe coating and the steel matrix, with the result that the weldabilitydecreases.

If Pb, Sb or Ni, sometimes contained in trace amounts in a galvanizingbath, exists in the coating layer in a range of not more than 0.1% each,then there is no problem in terms of characteristics of the steel. Whenthe Fe eluates into the galvanizing bath or the matrix Fe is mixed intothe coating layer, if its amount is not more than 0.1% or so, there isno problem. Mg may be contained in a range not more than 5% to impartcorrosion resistance to the steel. Besides those elements mentionedabove, others are preferably Zn or unavoidable impurities.

Even with a Zn—Fe galvanneal coated steel sheet whose coating layer isgalvannealed (hereafter referred to as GA), the quantity of Al in thecoating layer is required to be 0.1˜1%. The reason is as follows. If theAl content is below 0.1%, the Fe—Zn galvannealing reaction progresses sorapidly in a galvannealing process that the coating adhesiondeteriorates; on the other hand, if the Al content is more than 1%, theFe—Al alloy layer, which is formed on the coating layer side near theinterface between the coating and the steel matrix, becomes so thickthat the Fe—Zn galvannealing reaction is retarded. The desirable Alconcentration is not more than 0.3%. With a Zn—Fe galvanneal coatedsteel sheet whose coating layer is galvannealed, if the Fe content inthe coating layer is under 7%, a soft Zn—Fe galvanneal layer is formed,resulting in a lower slidability. Or, if the Fe content is over 15%, ahard and brittle Fe—Zn galvanneal layer is formed on the coating layerside near the interface between the steel matrix and the coating layer,and the result is a poor coating adhesion. For this reason, the Fecontent in the Zn—Fe galvanneal coated steel sheet is preferably 7˜15%.If Pb, Sb and Ni are contained in amounts of not more than 0.1% each,there is no problem in soldering properties. If Mg is contained in arange of not more than 5% to secure corrosion resistance, there is noproblem at all. The rest preferably consists of Zn and unavoidableimpurities.

For hot-dip galvanizing, a well-known method can be applied. Forexample, preferably, the bath temperature is 440˜500° C. and the Alconcentration in the bath is 0.10˜0.20% when a galvannealing process isperformed which will be described later, or 0.14˜0.24% when agalvannealing process is not performed. Mg may be contained in the bathfor better corrosion resistance.

After hot-dip galvanizing, if the coating layer undergoes agalvannealing process in a temperature range of 460˜550° C., this ismost desirable. If the temperature is under 460° C., the galvannealingreaction progresses slowly. Or if the temperature is above 550° C.,excess galvannealing occurs; therefore, a hard and brittle Zn—Fegalvanneal layer is formed in large amounts at the interface of thematrix, causing the coating adhesion to deteriorate. Furthermore, if thesteel is a steel with a retained austenite phase formed in the steel,when the galvannealing temperature is higher than 550° C., the retainedaustenite decomposes, and the strength-ductility balance tends todeteriorate. The coating layer weight has not been set, but from aviewpoint of securing the corrosion resistance and controlling theprecision of coating weight, it is preferably not less than 10 g/m², andnot more than 120 g/m² from a viewpoint of workability.

EMBODIMENT

Slabs of compositions shown in Table 2 are heated and held in an N₂atmosphere of O₂ densities as shown in Tables 3 and 4 in a heatingfurnace at temperatures and conditions shown in Tables 3 and 4, andsubsequently hot-rolled to a thickness of 2.8 mm and are coiled at540˜600° C. Subsequently, the skin scale was removed by acid cleaning.Some of hot-rolled steel sheets which have been acid-cleaned arecold-rolled into cold-rolled steel sheets with a thickness of 1.6 mm.Hot-rolled and cold rolled steel sheets obtained undergo primaryannealing and overaging under conditions shown in Tables 3 and 4, andare hot-dip galvanized in a molten Zn bath at a bath temperature of 460°C. Or, if they are subjected to secondary annealing, they receiveprimary annealing and cooling, and then acid-cleaned in a 5% HClsolution at 60° C. After this, the steel sheets are heated at secondaryannealing temperatures shown in Tables 3 and 4, and continuously theyare hot-dip galvanized in a molten Zn bath at a bath temperature of 460°C.

The Al concentration in the molten Zn bath is adjusted as shown inTables 5 and 6. The coating weight is adjusted to 50±5 g/m² for singleside by gas wiping. When the coating layer is galvannealed, thegalvannealing process is performed at 460˜610° C.

Obtained hot-dip galvanized steel sheets are evaluated in terms ofexternal appearance, coating adhesion, and mechanical properties.Samples are taken from the produced steel sheets, and the thickness wasmeasured of the AlN precipitate layer formed on the matrix side rightunder the interface between the matrix and the coating layer, and the Alconcentration and the Fe concentration in the coating layer weremeasured. Investigation results are shown in Tables 5 and 6.

TABLE 2 Steel Chemical Composition (mass %) Symbol C Si Mn P S Al Ti NbMo Cu Ni Remarks A 0.080 0.60 2.0 0.010 0.008 0.4 — — — — — ProperExample B 0.100 0.03 1.6 0.010 0.007 1.6 — — — — — Proper Example C0.070 0.20 1.6 0.010 0.008 0.2 — — — — — Proper Example D 0.090 0.04 1.40.008 0.006 1.6 0.01 0.03 0.05 — — Proper Example E 0.080 0.06 1.2 0.0110.009 0.9 0.04 — 0.1 0.2 0.1 Proper Example F 0.060 0.03 1.7 0.007 0.0060.03 — — — — — Comparative Example G 0.070 0.50 2.2 0.013 0.009 0.3 — —— — — Proper Example H 0.090 0.25 1.8 0.010 0.008 0.21 — — — — — ProperExample I 0.077 0.04 1.4 0.009 0.009 1.5 0.04 — 0.11 0.19 0.09 ProperExample J 0.060 0.03 1.3 0.007 0.006 1.3 0.01 0.04 0.06 — — ProperExample K 0.080 0.02 1.8 0.011 0.007 2.2 — — — — — Proper Example L0.080 0.06 1.8 0.008 0.006 0.02 — — — — — Comparative Example

TABLE 3 AlN Precipitate Layer Slab Heating & Holding ConditionsThickness Secondary 0₂ Concen- Of AlN Ppt. Al Solid Primary AnnealingAnnealing Steel Value of tration In Layer In Soln. Pct. Anneal.Overaging Anneal. Sheet Steel Ht. & Hld. Ht. & Hld. LHS of Slab-HeatingHot-Rolled In AlN Ppt. Temp. Temp. Temp. No. Symbol Temp. (° C.) Time(min) Eq. (1) *1 Atm. (vol %) Sheet(μm) Layer(%) *2 (° C.) (° C.) (° C.)1 A 1200 30 4200 2 0.9 20 825 — — 2 A 1200 40 5600 2 4 20 825 — — 3 A1200 60 8400 2 9 20 825 — — 4 A 1200 70 9800 2 11 20 825 — 775 5 A 120060 8400 2 9 20 825 — — 6 A 1260 60 12000  2 18 20 825 450 — 7 B 1200 505500 2 7 22 825 — — 8 C 1200 70 10150  2 11 10 825 — — 9 D 1200 50 55002 7 22 825 — — 10 E 1200 60 7650 2 9 21 825 — — 11 A 1050 30 −300 2 0 —825 450 — 12 A 1050 30 −300 2 0 — 825 450 — 13 A 1250 40 7600 2 15 20825 450 — 14 A 1200 60 8400 2 9 20 825 450 — 15 A 1200 70 9800 2 11 20825 450 — 16 F 1090 40 1570 2 0 — 825 450 — *1: LHS value of Eq.(1) ={ht. & hld. temp. − (1050 + 25Al)} × ht. & hld. time *2: Al solid soln.(%) = Al concentration in matrix of AlN ppt. layer/Al concentration atcenter in sheet thickness direction × 100

TABLE 4 AlN Precipitate Layer Slab Heating & Holding ConditionsThickness 0₂ Concen- Of AlN Ppt. Al Solid Primary Annealing SecondarySteel Value of tration In Layer In Soln. Pct. Anneal. OveragingAnnealing Sheet Steel Ht. & Hld. Ht. & Hld. LHS of Slab-HeatingHot-Rolled In AlN Ppt. Temp. Temp. Anneal. No. Symbol Temp. (° C.) Time(min) Eq. (1) *1 Atm. (vol %) Sheet (μm) Layer (%) *2 (° C.) (° C.)Temp. (° C.) 17 G 1200 40 5700  4   0.3 25 820 450 — 18 G 1240 60 10950  6 8 25 810 400 — 19 G 1250 70 13475  12 22  25 825 460 — 20 G 1260 408100 10 8 25 825 — 775 21 G 1250 40 7700 10 8 25 810 450 — 22 G 1250 509625 10 9 25 825 450 — 23 H 1250 50   9737.5 10 10  10 825 450 — 24 I1240 50 7625 11 8 22 825 440 — 25 J 1250 50 8375 10 8 22 825 450 — 26 K1250 50 7250 10 8 25 800 460 — 27 G 1100 50 2125 10 0 — 825 440 — 28 G1250 10 1925 10 0 — 825 450 — 29 L 1200 50 7475  9 0 — 825 430 — 30 G1250 40 7700 25 0 — 825 440 — 31 G 1250 40 7700   0.5 0 — 825 450 — 32 G1250 40 7700 10 8 25 950 450 — 33 G 1260 40 8100  8 9 25 825 450 — 34 G1210 50 7625  9 8 25 825 460 — 35 G 1240 60 10950  10 12  25 825 450 —*1: LHS value of Eq. (1) = {ht. & hld. temp. − (1050 + 25Al)} × ht. &hld. time *2 : Al solid soln. % = Al concentration in matrix of AlN ppt.Layer/Al concentration at center in sheet thickness direction × 100

TABLE 5 Hot-Dip Galvanizing Steel Sheet Hot-Dip Galvanizing ConditionsAl Solid Al Fe Al Soln. Concen- Concen- Kind Of Concen- Galvan- Pct. Intration In tration In Steel Coating tration nealing Thickness AlN Ppt.Coat. Coat. Sheet Kind Of Base In Bath Temp. Of AlN Ppt. Layer LayerLayer No. Coating Sheet (mass %) (° C.) Layer (μm) (%) *2 (mass %) (mass%) 1 GA CR 0.13 510   0.5 20 0.2 10 2 GA CR 0.13 490 2 20 0.2  9 3 GA CR0.13 500 5 20 0.2 10 4 GA CAL 0.13 500 6 20 0.2 11 5 GI CR 0.18 — 5 20 0.40 — 6 GA HOT 0.13 500 18  20 0.2 11 7 GA CR 0.13 500 4 22 0.2 12 8GA CR 0.13 500 6 10 0.2 11 9 GA CR 0.13 500 4 22 0.2 10 10 GA CR 0.13500 5 21 0.2  9 11 GA CR 0.13 500 0 — 0.2  8 12 GI CR 0.18 — 0 —  0.40  0.4 13 GA CR 0.08 460 8 20  0.05  9 14 GA CR 0.22 520 5 20 1.1  6 15GA CR 0.13 610 6 20 0.2 16 16 GA CR 0.13 520 0 — 0.2 12 Steel EvaluationSheet External Coating Mech. No. Appearance Adhesion Char. 1 Δ Δ GoodInvent. Ex. 1 2 ◯ ◯ Good Invent. Ex. 2 3 ◯ ◯ Good Invent. Ex. 3 4 ◯ ◯Good Invent. Ex. 4 5 ◯ ◯ Good Invent. Ex. 5 6 ◯ ◯ Good Invent. Ex. 6 7 ◯◯ Good Invent. Ex. 7 8 ◯ ◯ Good Invent. Ex. 8 9 ◯ ◯ Good Invent. Ex. 910  ◯ ◯ Good Invent. Ex. 10 11  X X Good Comp. Ex. 1 12  X X Good Comp.Ex. 2 13  ◯ X Good Comp. Ex. 3 14  X *3 ◯ Good Comp. Ex. 4 15  ◯ X GoodComp. Ex. 5 16  ◯ ◯ No Comp. Ex. 6 Good *2: Al solid soln. (%) = Alconcentration in matrix of AlN ppt. layer/Al concentration at center insheet thickness direction × 100 *3: Unevenness in galvannealinq reaction

TABLE 6 Hot-Dip Galvanizing Conditions Al Hot-Dip Galvanizing SteelSheet Kind Of Concen- Galvan- Al Solid Fe Concen- Steel Coating trationnealing Thickness Soln. Pct. In Al Concentra- tration In Sheet Kind OfBase In Bath Temp. Of AlN Ppt. AlN Ppt. tion In Coat. Coat. Layer No.Coating Sheet (mass %) (° C.) Layer (μm) Layer (%) *2 Layer (mass %)(mass % ) 17 GA CR 0.13 500 0.2 25 0.21 10 Invent. Ex. 11 18 GA CR 0.13500 5 25 0.18  9 Invent. Ex. 12 19 GA CR 0.13 500 12 25 0.22 10 Invent.Ex. 13 20 GA CAL 0.13 500 5 25 0.19 11 Invent. Ex. 14 21 GI CR 0.18 — 525 0.40 — Invent. Ex. 15 22 GA HOT 0.13 500 9 25 0.21 11 Invent. Ex. 1623 GA CR 0.13 500 6 10 0.2  12 Invent. Ex. 17 24 GA CR 0.13 500 5 220.24 11 Invent. Ex. 18 25 GA CR 0.13 500 5 22 0.18 10 Invent. Ex. 19 26GA CR 0.13 500 5 25 0.2   9 Invent. Ex. 20 27 GA CR 0.13 500 0 — 0.19  8Comp. Ex. 7 28 GA CR 0.13 500 0 — 0.23 11 Comp. Ex. 8 29 GA CR 0.13 5000 — 0.2  12 Comp. Ex. 9 30 GA CR 0.13 500 0 — 0.21 11 Comp. Ex. 10 31 GACR 0.13 500 0 — 0.2  11 Comp. Ex. 11 32 GA CR 0.13 500 5 25 0.18   6.9Comp. Ex. 12 33 GA CR 0.09 500 5.5 25 0.05 17 Comp. Ex. 13 34 GA CR 0.21450 5 25 1.1   6 Comp. Ex. 14 35 GA CR 0.13 610 7 25 0.19 16 Comp. Ex.15 Steel Evaluation Sheet No. External Appearance Coating Adhesion Mech.Char. 17 Δ Δ Good Invent. Ex. 11 18 ◯ ◯ Good Invent. Ex. 12 19 ◯ ◯ GoodInvent. Ex. 13 20 ◯ ◯ Good Invent. Ex. 14 21 ◯ ◯ Good Invent. Ex. 15 22◯ ◯ Good Invent. Ex. 16 23 ◯ ◯ Good Invent. Ex. 17 24 ◯ ◯ Good Invent.Ex. 18 25 ◯ ◯ Good Invent. Ex. 19 26 ◯ ◯ Good Invent. Ex. 20 27 X X GoodComp. Ex. 7 28 X X Good Comp. Ex. 8 29 ◯ ◯ No Good Comp. Ex. 9 30 X *4 XGood Comp. Ex. 10 31 X X Good Comp. Ex. 11 32 X X No Good Comp. Ex. 1233 ◯ X Good Comp. Ex. 13 34 X *3 ◯ Good Comp. Ex. 14 35 Δ X No GoodComp. Ex. 15 *2: Al solid soln. (%) = Al concentration in matrix of AlNppt. layer/Al concentration at center in sheet thickness direction × 100*3: Unevenness in galvannealing reaction *4: Surface becomes rough

In Tables 5 and 6, as coating kinds, a hot-dip galvanized coating layerthat has been galvannealed is designated as GA and a hot-dip galvanizedcoating layer that has not been galvannealed is designated as GI. Withregard to kinds of base sheets for coating, a case where a hot-rolledsteel sheet is used is designated as HOT, a case where a cold-rolledsteel sheet annealed once is used is designated as CR, and a case wherea cold-rolled steel sheet subjected to annealing—acid cleaning—reheatingis used is designated as CAL.

The external appearance was evaluated by visually checking thegalvanizability with reference to criteria as follows.

-   ∘: Without any “bare-spot”-   Δ: Generally no problem even though there are some “bare spots”-   X: “bare” spots occurred conspicuously

The coating layer adhesion was evaluated as follows. A cellophane tapewas attached to the surface of a specimen, the tape-attached side wasbent 90 degrees and bent back, the tape was peeled off, the peeled-offamount of the coating per unit length was measured in terms of counts ofZn by a fluorescent X-ray method. Samples ranked first or second wereclassified as good (∘, Δ) and those ranked third and so on wereclassified as defective (X) by referring to the criteria in Table 1.

A hot-dip galvanized steel sheet (GI) that has not been galvannealed wasput to a ball impact test, and a cellophane tape was attached to aprocessed part and the tape was peeled off. Whether the coating layerwas peeled off or not was evaluated according to criteria as follows.

-   ∘: The coating layer did not peel off.-   Δ: The coating layer peeled off a little.-   X: The coating layer peeled off conspicuously.

In evaluation of mechanical properties, JIS-No.5 tension test specimenswere collected, and from measured tensile strength TS(MPa) andelongation El (%), if a result of TS×El was not less than 20000 MPa·%,this was taken to represent a good strength-ductility balance and thehot-dip galvanized steel was determined to have a favorable mechanicalproperties.

With regard to the Al concentration in the coating layer, the coatingwas dissolved in an alkali, such as NaOH or KOH, or in an acid, such asHCl or H₂SO₄, added with an inhibitor, and its solution was analyzed andthe concentration of Al was determined by a plasma emission spectrometer(ICP).

Likewise, the Fe concentration in the coating layer was measured by ICPby analysis and quantitative determination of Fe.

The thickness of the AlN precipitate layer was obtained by analyzing thesectional surface of a galvanized steel sheet by EPMA, and measuring thethickness of a region where there were precipitates of AlN and the Alconcentration in the matrix area is lower than the central portion ofthe steel sheet. The Al concentration of the matrix area in the AlNprecipitate layer was obtained by analyzing the above-mentionedinsoluble residues.

From Tables 5 and 6, the hot-dip galvanized steel sheet (GA or GI) is Δor ∘ in evaluation of the external appearance and Δ or ∘ in evaluationof coating adhesion; therefore, it is understood that this galvanizedsteel sheet has favorable galvanizability and coating adhesion.Furthermore, in respect to mechanical properties, the steel sheet showsa favorable strength-ductility balance of 20000 MPa·% or higher.

In contrast, in comparative examples 1, 2, 7, 8, 10 and 11, there is notAlN precipitate layer, and therefore the external appearance and thecoating adhesion are poor. In comparative examples 3 and 13, in whichthe Al concentration is low in the coating, the coating adhesion ispoor. In comparative examples 4, 5, 14 and 15 of Zn—Fe galvanneal coatedsteel sheet, the Al concentration in the coating layer is high incomparative examples 4 and 14 and the Fe concentration in the coatinglayer is low, so that there is unevenness in the galvannealing results.In comparative examples 5 and 15, because of the high Fe concentrationin the coating layer, the coating adhesion is insufficient. Incomparative examples 6 and 9, in which steel sheets of low Al contentwere used as base sheets, it is obvious that the mechanical propertiesare inferior. In comparative example 12, because the primary annealingtemperature is too high, the galvannealing reaction in the coating layerdid not progress sufficiently, and moreover, the coating adhesion andmechanical properties are deficient.

(2) SECOND EMBODIMENT

Description will now be made of a chemical composition of base steelsheet to undergo electroplating or chemical treatment according to asecond embodiment of the present invention. Also in the secondembodiment, the contents of elements in the steel are expressed simplyby percent (%) which, however, should all be construed to mean “inpercent by mass (mass %)”.

(Al: not Less than 0.1% and Under 3%)

In this embodiment, as in the first embodiment, steels with an Alcontent of not less than 0.1% are used. The reason is as follow: So longas the Al content is under 0.1%, because the surface-segregated amountof Al is low, there are not much of problems in the electrocoatingadhesion and the unevenness of electroplated coating or chemicaltreatment coating, or the external appearance, and AlN is not formed,either. Also in this embodiment, with a view to securing astrength-ductility balance, preferably, the chemical compositioncontains retained austenite, and if the Al content is under 0.1%, theretained austenite is unstable; therefore, with a view to obtaining abetter strength-ductility balance of the steel sheet, it is requiredthat not less than 0.1% of Al should be contained in the steel sheet.Note that in a steel sheet with an Al content of not less than 3.0 mass%, even if AlN is formed in the surface layer of a steel sheet, thesurface segregated Al is formed in large amounts during annealing,making it difficult to secure a better coating adhesion despite the bestefforts that could be made to improve the method for forming a nitridelayer; therefore, the Al content in the steel is set under 3.0%.

(One or Two Kinds Out of not Less than 0.1% of Si and not Less than 0.5%of Mn)

For the same reason as in the first embodiment, one or two kinds out ofSi and Mn are set in the ranges mentioned above. (One or two kindsselected out of not less 0.01% and not more than 1% of Mo, and not lessthan 0.005% and not more than 0.2% of Nb)

For the same reason as in the first embodiment, one or two kinds out ofMo and Nb are set in the ranges mentioned above. (Not less than 0.01%and not more than 0.5% of Cu, not less than 0.01% and not more than 1%of Ni, not less than 0.01% and not more than 1% of Mo)

By adding Cu, Ni and Mo, the coating adhesion of the steel sheet isimproved. The mechanism of improving the electrocoating adhesion andchemical properties by adding Cu, Ni and Mo has not been clarified. But,the present inventors have confirmed that when those elements are addedtogether, but not singly, the internal oxidation of Al is promotedduring annealing, and therefore Al is prevented from segregating in thesurface, thus improving the coating adhesion.

With regard to other components, considering production cost and alsoworkability when a steel sheet is used as an automotive steel sheet,preferably, 0.005˜0.25% of C, 0.001˜0.20% of P, and 0.0001˜0.10% of Sshould be contained. Besides these elements, to control astrength-ductility balance of the steel sheet, it is permissible tocontain not more than 0.15% of Ti, not more than 1% of Cr and0.001˜0.005% of B. The rest consists of Fe and unavoidable impurities.

Description will be made of the AlN precipitate layer formed in thesurface layer of a steel sheet, which is an important point of thisembodiment.

In this embodiment, as in the first embodiment, the AlN precipitatelayer is formed in the surface layer of the steel sheet, and fixed inthe form of nitride in the matrix, the Al in the steel of the surfacelayer is prevented from diffusing into the surface layer of the steelsheet in annealing or acid cleaning.

Though the reason has not been clarified, it has been confirmed that thepresence of the AlN precipitate layer has the effect of inhibiting Siand Mn as readily oxidizable elements other than Al from segregating atthe surface of the steel during annealing. Therefore, even with steelswhich contain relatively large amounts of Si and Mn and which tend tocause a poor coating adhesion or bare spots, the presence of an AlNprecipitate layer secures better electroplated coating properties andcoating adhesion.

The cross section of a steel sheet having an AlN precipitate layerformed in the surface layer is the same as shown in FIG. 1 of the firstembodiment (the result observed by an electron microscope (SEM), and thestate of presence of Al by EPMA is the same as shown in FIG. 2 of thefirst embodiment. Therefore, the Al exists as nitride in a columnar orangular form and is distributed in a region 10˜20 μm deep from theinterface of the steel matrix, and it is obvious that in that portion ofthe region where AlN did not precipitate out, there is not much of solidsolution of Al. This region corresponds to the AlN precipitate layer.Therefore, the solid solution of Al is inhibited from diffusing fromthis region to the surface during annealing, and therefore the coatingadhesion does not deteriorate. It is understood that in an area muchdeeper than that region, the presence of nitride is not recognized, butAl exists mostly in a solid solution phase.

For the same reason as in the first embodiment, in this embodiment, thethickness of the AlN precipitate layer is preferably not less than 1 μmand not more than 100 μm.

Description will now be made of a preferred method for manufacturing acoated steel sheet according to the present invention. Like with anordinary steel sheet, this steel sheet (a base sheet for anelectroplating or chemical treatment process) is manufactured by heatingand holding a cast steel slab (continuously-cast slab for example) for apredetermined time and having the slab go through a hot-rolling processor, if necessary, further subjecting it to a cold-rolling process. Inthis invention, however, in order to prevent Al from segregating at thesurface layer in the annealing process before electroplating or chemicaltreatment, an AlN precipitate layer is previously formed in the surfacelayer of the steel sheet before annealing or after acid cleaning.

The present inventors, to form this AlN precipitate layer, carried outthe above-mentioned heating and holding of the cast steel slab in anatmosphere containing not less than 1 vol % and not more than 20 vol %of O₂ and not less than 70 vol % of N₂ like in the first embodiment.

By carrying out the above-mentioned heating and holding of the caststeel slab in an N₂-containing atmosphere at a raised holdingtemperature and for a prolonged holding time, the Al in the surfacelayer of the steel slab can be nitirided. In this case, if the Alcontent is high in the cast slab, the heating and holding time necessaryfor nitridation is prolonged accordingly. Regarding steels with varyingAl contents, investigation was made into the effects that heating andholding time and temperature before hot rolling have on theelectroplated coating adhesion and the chemical properties, and detailsare described as follows.

A cast steel slab of a composition containing 0.1˜3% of Al, 0.5% of Siand 2.2% of Mo was heated and held in an atmosphere of 70 vol % of O₂and the rest consisting of of N₂ was hot-rolled into a steel sheet of athickness of 2.8 mm. The oxide scale on the surface of an obtainedhot-rolled steel sheet was removed by acid cleaning, and then the steelsheet was cold-rolled to a 1.6 mm thickness, and annealed at 810˜825°C., overaged at 400˜460° C., and then subjected to electro-galvanizingand Zn phosphating by well-known methods, respectively.

An obtained electro-galvanized steel sheet was evaluated in coatingadhesion by an OT bending test as follows.

In the OT bending test, an electro-galvanized steel sheet was folded intwofold without leaving no gap with the side under evaluation of thecoating adhesion facing outside, a cellophane tape was attached to thefolded portion and peeled off, and the amount of the coating adhering tothe cellophane tape was visually inspected. Evaluation was madeaccording to the criterion 1.

(Criterion 1):

-   ∘: The coating did not peel off.-   Δ: The coating slightly peeled off to a level of no problem.-   X: The coating peeled off conspicuously.

An obtained zinc-phosphated steel sheet was visually inspected to see ifthere was any unevenness in adhesion of the zinc phosphate film, andevaluated according to criterion 2 as follows.

(Criterion 2)

-   ∘: The coating weight is free of unevenness.-   Δ: The coating weight was slightly uneven but no problem.-   X: The coating weight was conspicuously uneven.

In those evaluations, if those which were evaluated as ∘ or Δ in both ofelectroplated coating adhesion and chemical properties are classified as∘ and those which were evaluated as X in one or both of electroplatedcoating adhesion and chemical properties are classified as X, then theevaluation results of this embodiment are the same as those in FIGS. 3Ato 3D showing the evaluation of the coating adhesion in the firstembodiment.

Therefore, as is obvious from FIGS. 3A to 3D, by heating and holding acast steel sheet before hot rolling under the conditions that theheating and holding temperature, the heating and holding time and the Alcontent all meet Eq. (1), in other words, under the conditions that fallwithin the upper area of the border line between the ∘-mark area and theX-mark area, an electro-galvanized steel sheet can be manufactured withexcellent coating adhesion.{Heating & holding temperature (° C.)−(1050+25Al)}×heating & holdingtime (min)≧3000  (1)where Al denotes the Al content (mass %) in the steel.

It could be confirmed that when Eq. (1) is satisfied, the Al precipitatelayer is formed in the surface layer of the steel sheet.

As has been described, with a cast steel sheet of a composition thatcontains not less than 0.1% and under 3% of Al, by heating and holdingbefore hot rolling under conditions that meet Eq. (1) in an atmospherecontaining not less than 1 vol % and not more than 20 vol % of O₂, asteel sheet having an AlN precipitate layer formed in the surface layercan be manufactured, and even with a steel sheet containing readilyoxidizable elements such as Si and Mn, besides Al, it is possible toachieve better electro-galvanized coating adhesion and chemicalproperties.

A hot-rolled steel sheet that has been hot-rolled after heated and heldunder the above-mentioned conditions is, after acid cleaning or afteracid cleaning, cold rolling, and annealing, subjected to electroplatingand chemical treatment processes.

In this embodiment, acid cleaning after hot rolling is done to removethe oxide scale formed on the surface, but acid-cleaning conditions arenot specified. However, because an AlN precipitate layer needs toremain, consideration is required to prevent the matrix from dissolvingin large amounts during acid cleaning. The acid to be used for cleaningis preferably hydrochloric acid, but other acids such as sulfuric acidmay be used. The acid concentration is preferably 1˜20 mass %. Toprevent the matrix from dissolving in large amounts, an inhibitor(dissolution inhibitor) may be added in the acid cleaning liquid.

In this embodiment, cold rolling is carried out when necessary tocontrol the mechanical properties and the sheet thickness of a finalproduct. Cold rolling is preferably performed with a draft of not lessthan 30% to promote recrystallization in subsequent annealing. Note thatwhen the draft is not less than 80%, load on the rolling mill is solarge that rolling becomes difficult, so that the draft is preferably30˜80%.

Further in this embodiment, annealing may be carried out by a well-knowncontinuous annealing method. Annealing may be performed not only oncold-rolled steel sheets but also on hot-rolled steel sheets. To achievea better balance between strength and ductility of steel sheet, whenboth annealing by a continuous annealing method and electro-galvanizingare carried out continuously, it is preferable to perform averaging at350˜500° C. for two minutes or longer after intercritical heating, andwhile having bainite transformation to progress, make the carbonsegregate in the austenite, and subsequently carry outelectro-galvanizing. When electro-galvanizing is carried out afterprimary annealing, cooling, acid cleaning, and secondary annealing, itis preferable to perform intercritical heating by primary annealing,then quenching the steel sheet down to 300° C. or lower at a rate of 40°C./s or higher to thereby form a hardened structure composed offerrite/martensite phases, and just before galvanizing, and thentempering the steel sheet by heating to 725˜840° C. and cooling at arate of 5° C./s or higher to thereby form a composite structure ferrite,tempered martensite and retained austenite.

The technology for forming the AlN precipitate layer, which has beendescribed, is to control conditions for heating and holding before hotrolling. However, to manufacture a coated steel sheet according to thisembodiment, this technology for controlling the heating and holdingconditions may not be adopted. For example, the above-mentioned coatedcan be manufactured by a method of annealing in an atmosphere ofnitriding elements, such as an H₂—N₂ system gas, mixed with traceamounts of CO and NH₃ in the annealing process.

As electroplating applied to a steel sheet for coating according to thisembodiment, electro galvanized coating, in which the chief component isZn, is most suitable. For example, besides pure-zinc electroplating,zinc-alloy electroplating that contains elements, such as Fe, Ni, Co andMo, may be cited. In addition, it is possible to use a zinc-basedelectroplated coating which has inorganic compounds or organic compoundsdispersed or separated out in a zinc-based electroplated coating. As achemical treatment process, an ordinary method, such as zinc-phosphatingmay be applied.

A coated steel sheet according to this embodiment, though subjected toelectroplated coating or chemical treatment, the coating adhesion,coating weight and crawling of the electroplated coating, and thecoarsening of crystalline grains by chemical treatment are improved to agreat extent.

EMBODIMENT

A cast steel slab of a composition shown in Table 7 was heated and heldin a heating furnace under the conditions shown in Table 8 andsubsequently hot-rolled to a thickness of 2.8 mm and wound up in a coilat 540˜600° C. After this, the skin scale was removed from the strip byacid cleaning. Some of acid-cleaned hot-rolled steel sheet iscold-rolled into a cold-rolled strip with a thickness of 1.6 mm,annealed at 800˜850° C., and overaged at 400˜500° C., and cooled.

An obtained hot-rolled strip or cold-rolled strip as a base sheet wascoated by any one of Zn phosphating, pure-zinc electroplated coating,electro zinc-nickel alloy coating, and electro zinc-iron coating by awell-known method. Base sheets were measured to determine the thicknessof the AlN precipitate layer and the solid solution rate of Al in theAlN precipitate layer. An electroplated steel sheet was subjected to theOT bending test mentioned above, and the coating adhesion was evaluated.The electroplated coating property or the chemical properties wasdecided from external appearance by visual examination to see if therewas external unevenness, such as unevenness of coating weight, andevaluated according to Criterion 2.

As for mechanical properties, JIS-specified No.5 tension test specimenswere taken from the strip, and tensile test was conducted to measuretensile strength (TS (MPa)) and elongation (El (%)). From measuredvalues, TS×El was obtained, and if the value was 20,000 (MPa·%) orlarger, this was decided to show a favorable strength-ductility balanceof the steel sheet, and the mechanical properties were evaluated asgood.

From FIG. 8, it is obvious that in embodiments of the invention in whichelectroplating was performed, steel sheets are superior in the coatingadhesion and the external appearance, and exhibit excellent mechanicalproperties. It is also evident that steel sheets in embodiments of theinvention in which chemical treatment was carried out show superbexternal appearance and offers favorable mechanical properties.

TABLE 7 Steel Chemical Composition (mass %) Symbol C Si Mn P S Al Ti NbMo Cu Ni Remarks A 0.080 0.60 2.0 0.010 0.008 0.4 Proper Ex. B 0.1000.03 1.6 0.010 0.007 1.6 Proper Ex. C 0.070 0.20 1.6 0.010 0.008 0.2Proper Ex. D 0.090 0.04 1.4 0.008 0.006 1.6 0.01 0.03 0.05 Proper Ex. E0.080 0.06 1.2 0.011 0.009 0.9 0.04 0.1 0.2 0.1 Proper Ex. F 0.060 0.031.7 0.00 70.006 0.03 Comparative Ex.

TABLE 8 Slab Heating & Holding Conditions Steel Ht. & Hld. Value of 0₂Concentration Al Solid Soln. Sheet Steel Ht. & Hld. Time LHS of InSlab-Heating Thickness Of AlN Pct. In AlN Ppt. No. Symbol Temp. (° C.)(min) Eq. (1) *1 Atm. (vol %) Ppt. Layer (μm) Layer (%) *2 1 A 1200 304200 1.5 0.3 20 Invent. Ex. 1 2 A 1200 40 5600 1.5 1 20 Invent. Ex. 2 3A 1250 40 7600 1.5 4 20 Invent. Ex. 3 4 A 1250 40 7600 1.5 4 20 Invent.Ex. 4 5 A 1250 40 7600 10 6 20 Invent. Ex. 5 6 A 1250 40 7600 8 5 20Invent. Ex. 6 7 B 1250 40 7600 10 3 22 Invent. Ex. 7 8 C 1250 40 76001.5 5 10 Invent. Ex. 8 9 D 1250 40 7600 1.5 4 22 Invent. Ex. 9 10 E 125040 7600 15 6 21 Invent. Ex. 10 11 A 1250 40 7600 0.1 0 — Comp. Ex. 1 12F 1250 40 7600 3 0 — Comp. Ex. 2 13 A 1200 40 5600 1.5 2 20 Invent. Ex.11 14 A 1200 40 5600 1.5 2 20 Invent. Ex. 12 15 A 1050 30 −300 2 0 —Comp. Ex. 3 Evaluation Steel Kind Of Coating/Chemical External CoatingSheet No. Base Sheet Conv. Kind *3 Appearance Adhesion Mech. Char. 1Cold Zn Δ Δ Good Invent. Ex. 1 2 Cold Zn ◯ ◯ Good Invent. Ex. 2 3 ColdZn—Ni ◯ ◯ Good Invent. Ex. 3 4 Cold Zn—Fe ◯ ◯ Good Invent. Ex. 4 5 ColdZn phos Ch. Treat ◯ — Good Invent. Ex. 5 6 Cold Zn ◯ ◯ Good Invent. Ex.6 7 Cold Zn ◯ ◯ Good Invent. Ex. 7 8 Cold Zn ◯ ◯ Good Invent. Ex. 8 9Cold Zn ◯ ◯ Good Invent. Ex. 9 10  Cold Zn ◯ ◯ Good Invent. Ex. 10 11 Cold Zn X X Good Comp. Ex. 1 12  Cold Zn ◯ ◯ No Good Comp. Ex. 2 13  HotZn ◯ ◯ Good Invent. Ex. 11 14  Hot Zn phos Ch. Treat ◯ — Good Invent.Ex. 12 15  Cold Zn X X Good Comp. Ex. 3 *1: LHS value of Eq.(1) = {ht. &hld. temp. − (1050 + 25Al)} × ht. & hld. time *2: Al solid soln. (%) =Al concentration in matrix of AlN ppt. Layer/Al concentration at centerin sheet thickness direction × 100 *3: Zn: pure zinc electro coating;Zn—Ni: Zn—Ni electro coating; Zn—Fe: Zn—Fe electro coating

(3) THIRD EMBODIMENT

Description will be made of a chemical composition of a high-strengthsteel sheet and a base sheet for a high-strength hot-dip galvanizedsteel sheet according to a third embodiment of the present invention.Also in the third embodiment, the contents of elements in the steel areexpressed simply by percent (%) which, however, should all be construedto mean “in percents by mass (mass %)”.

(Not Less than 0.1% and Under 3.0% of Al)

For the same reason as in the first embodiment, the Al content is in theabove-mentioned range.

(C: 0.03˜0.25%)

The C content needs to be not less than 0.03% to secure a desiredstructure. However, a large amount of C degrades weldability, andtherefore the C content should be limited to more than 0.25%.

(Si: 0.001˜1.0%)

To obtain desired strength and structure, not less than 0.001% of Si isadded. If Si, like with Al, is present as an internal oxide in thesurface layer, the problem of surface treatability can be avoided.However, If the Si content in the steel is over 1.0%, even though Si ispresent in the surface layer but as its oxide in an inner area of thesteel sheet, the post-painting corrosion resistance of the steel sheetwhich was coated is low. For this reason, the upper limit of Si contentis 1.0%.

(Mn: 0.5˜3.0%)

To obtain a desired strength, Mn is added in an amount of 0.5% or more.However, if the added amount is over 3%, the weldability deteriorates,so that the Mn content is limited to not more than 3.0%.

(P: 0.001˜0.10%)

Though P is an element which is used to achieve high strength withoutaggravating deep drawing property; however, the addition of an excessiveamount of P delays a galvannealing reaction or deteriorates secondaryfabrication embrittlement, so that the P content is limited to not morethan 0.10%. The lower limit is 0.001%, a level of unavoidable impuritycontent.

Steel sheets according to the present invention, in addition to theindispensable elements mentioned above, may contain the followingcomponents where necessary.

(Mo: 0.01˜1.0%, Nb: 0.005˜0.2%)

Mo and Nb reduce the grain size of the matrix structure and delay therecrystallization, to thereby promote the internal oxidation of Al inthe temperature rising process, and are preferably added to obtainbetter coating property and coating adhesion. When Mo and Nb are addedtogether, however, if the Mo content is over 1.0%, the surfaceproperties of a hot-rolled steel sheet shows a tendency to deteriorate,or if the Mo content is under 0.01%, the intended effect is less likelyto be obtained. On the other hand, when Nb is added at a content of over0.2%, the hardness tends to rise and the ductility tends to deteriorate,but if the content is under 0.005%, the effect is small. Therefore,preferably, the Mo content is 0.01˜1.0%, and the Nb content is0.005˜0.2%. (Cu: 0.01˜0.5% ; Ni: 0.01˜1.0% ; Mo: 0.01˜1.0%)

Cu, Ni and Mo are elements desirable for obtaining better coatingadhesion, and only when they are added together, the internal oxidationis promoted during annealing, and therefore they are prevented fromsegregating in the surface, thus improving the coating adhesion.However, when they are in large amounts, there is a tendency for thesurface properties of the steel sheet to deteriorate, so that theiradded amounts need to meet the conditions: 0.01˜0.5% of Cu, 0.01˜1.0% ofNi, and 0.01˜1.0% of Mo.

As for other chemical components, considering an application as steelsheets for automobiles, to improve the strength-ductility balance, otherelements may be added when necessary in the following ranges: not morethan 0.15% of Ti, not more than 1% of Cr, and 0.001˜0.005% of B.

The rest other than the above-mentioned elements are preferably Fe andunavoidable impurities.

Description will next be made of the internal oxide layer to bepossessed by a high strength steel sheet and a high-strength hot-dipgalvanized steel sheet according to this embodiment.

In this embodiment, an oxide layer needs to be formed not in the surfaceof a steel sheet, but as a so-called internal oxide layer right belowthe surface of the matrix. As the amount of the oxide existing in thesurface of a steel sheet increases, not only the surface-treatabilityand the coating adhesion but also the weldability and post-paintingcorrosion resistance deteriorate. The region right below the surface(surface layer) at which the internal oxide layer preferably extendsgenerally in a range of 0.1˜100 μm from the surface of the steel sheet.If the thickness of the region where the Al oxide exists is less than0.1 μm, the formed amount of the oxide is so little that the surfaceoxidation of Al cannot be inhibited. On the other hand, if the range isgreater than 100 μm, there is a worry that the mechanical properties ofthe steel sheet itself deteriorates.

By making the internal oxidation take place, the Al oxide not onlyexists as the internal oxidation layer in the crystalline grains rightbelow the surface of the matrix, but also comes to exist in largeamounts in the grain boundaries. The Al oxides existing at the grainboundaries have the effect of inhibiting the corrosion reaction liableto progress from the grain boundaries. The oxides existing in the grainsalso have the effect of inhibiting the corrosion reaction fromprogressing from the grain boundaries into the grains. The details ofthis mechanism are unknown. The above-mentioned effects are enhanced bycoexistence of other oxides; therefore, to have the Al oxides coexistwith oxides of Fe, Si, Mn and P or the like is desirable for theimprovement of the corrosion resistance. Among the oxides of thoseelements are SiO₂, MnO, FeSiO₃, Fe₂SiO₄, MnSiO₃, Mn₂SiO₄ and P₂O₅.

The coexistence of those oxides contributes to the outstanding coatingadhesion during press working. It is presumed that the oxide layersexisting in adequate amounts serve to absorb compressive stresses inpress working. Therefore, the presence of an Al oxide and other oxidesis effective in additionally improving the coating adhesion.

Moreover, the presence of the Al oxide as the internal oxide layersecures an improved spot welding property. The reason is considered asfollows. Because the Al oxide that worsens the weldability is fixed asthe oxide in the steel and does not exist in the surface of the steelsheet, a substantive amount of Al solid solution is decreased and theweldability is thereby improved.

As has been described, in a steel sheet according to this embodiment,the internal oxide layer is required to exist right below the surface ofthe steel sheet, and to obtain the above-mentioned effects, the amountof oxides needs to be 0.01 g/m² for each side. However, when the amountof oxides is over 1.0 g/m², the internal oxidation progresses so much asto deteriorate the surface treatability and the coating adhesion, andmoreover the roughened surface worsens the external appearance anddegrades the corrosion resistance.

Note that the amount of the internal oxides can be known by measuringthe amount of oxygen in the steel; however, if a coated layer hasalready been formed, such as a coating layer, then the amount of oxygenin the steel is measured after the coated surface is removed. As for amethod for removing the coating layer, it is possible to use a solutionconsisting of an aqueous solution of 20 wt % of NaOH and 10 wt % oftriethanolamine and an aqueous solution of 35 wt % of H₂O₂ mixed in avolume ratio of 195:7 or a dilute HCl solution containing an inhibitor.Or, other acid or alkali may be used. Note that attention should betaken to prevent the surface of the steel sheet from oxidizing after thecoating layer has been removed. When measuring the amount of oxygen inthe steel, the amount of oxygen in the base sheet for the steel sheetneeds to be subtracted; therefore, after the coating layer has beenremoved, the amount of oxygen of only the surface layer is calculated bysubtracting the amount of oxygen existing in the steel obtained from aspecimen, the surfaces of both sides of which have been removed by 100μm or more from the surface by mechanical grinding, and then byconverting the calculated amount of oxygen into that per unit area, theinternal oxide amount is obtained. The amount of oxygen in the steel canbe obtained by measuring by “Impulse Furnace Melting-InfraredAbsorption”, for example. The kinds of oxides can be identified by usingextraction replicas of the cross section of the steel sheet, and byanalyzing them by TEM observation and EDX. If the amount of oxidesformed is so small and it is difficult to identify, the Br₂-methanolmethod may be used to extract and ICP analysis may be employed.

The above-mentioned internal oxide layer can be formed by setting acoiling temperature (CT) after hot-rolling at a high temperature of notlower than 640° C., or by annealing on a continuous annealing line (CAL)or on a continuous hot-dip galvanizing line (CGL), and in this processby setting the dew point (DP) of the atmosphere in the annealing furnaceon a rather higher side. In the former case, the internal oxide layer isformed by oxygen supplied from the skin scale, or in the latter case,the internal oxide layer is formed by oxygen formed when H₂O in theatmosphere decomposes at the surface of the steel sheet. Therefore, whenthe coiling temperature CT after hot rolling has to be low, the internaloxides may be formed in the annealing furnace. Above all else, Al isliable to form internal oxides, so oxidation occurs even at an ordinarydew point of about (DP=−40˜−30° C.), and even when an added amount is asmuch as close to 2%, surface segregation can be impeded sufficiently.However, when the dew point is below −50° C., internal oxidation becomesless liable to occur. Therefore, when a steel sheet is coiled at a highcoiling temperature CT, the dew point of the atmosphere in annealingmatters little, but when a steel sheet is coiled at a low CT, the dewpoint preferably not lower than −45° C., and more preferably not lowerthan −40° C. With regard to Si, Mn and P in the surface layer, internaloxides of those elements are formed by setting a high CT or bycontrolling the dew point in continuous annealing.

A high strength steel sheet according to the present invention can bemanufactured by using a high CT or by controlling the dew point incontinuous annealing as described above. Moreover, if a high strengthsteel sheet according to the present invention is hot-dip galvanized, ahot-dip galvanized steel sheet according to this embodiment can bemanufactured.

Next, description will be made of suitable conditions for annealing andhot-dip galvanizing when manufacturing steel sheet according to thisembodiment.

With regard to annealing conditions on a continuous annealing line(CAL), it is preferable from a viewpoint of obtaining high strengththat, after recrystallization is made to occur by heating the steelsheet to an intercritical temperature (α+γ), the steel sheet is overagedat 350˜500° C. for two minutes or longer to make C segregate in theaustenite, and bainite transformation is made to occur. When hot-dipgalvanizing a steel sheet, hot-dip galvanizing is carried out on a steelsheet on which recrystallization and averaging have been carried out, orafter the above-mentioned intercritical annealing has been done on acontinuous hot-dip galvanizing line (CGL), subsequently a hot-dipgalvanizing process may be performed. In the category of hot-dipgalvanized steel sheets, a hot-dip galvanized steel sheet featuring highstrength and high ductility can be obtained by performing theabove-mentioned intercritical annealing on a steel sheet on a CAL line,quenching 300° C. or lower at a rate of 40° C./sec or higher to producea hardened structure consisting of ferrite and martensite, and thenagain annealing the steel sheet at 725˜840° C. on the CGL line andcooling at a rate of 5° C./sec to obtain a steel sheet of a compositestructure of ferrite, tempered martensite and retained austenite, andsubsequently hot-dip galvanizing the steel sheet, which is the so-calledtwo-step annealing method.

As the annealing furnaces for CAL and CGL, all radiant heating tube(RTH) furnaces of so-called full-reducing atmosphere may be used. Forthe temperature-rising process, it is possible to use anon-oxidizing-atmosphere heating furnace (NOF) or a direct-firingfurnace (DFF).

As for a method for applying this embodiment to a hot-dip galvanizedsteel sheet, a well-known method may be used. For example, preferably,the bath temperature is 440˜500° C., and the Al concentration in thebath is 0.10˜0.20% when a galvannealing process is carried out, or about0.14˜0.24% when a galvannealing process is not carried out. If the Alconcentration is too low, the coating adhesion is inferior in both casesmentioned above. On the other hand, if the Al concentration is too high,the weldability is poor when the galvannealing process is not executedor the galvannealing reaction is delayed when the galvannealing processis carried out. To improve the corrosion resistance, Mg may be added tothe galvanizing bath. The coating weight is not specified, but from aviewpoint of securing the corrosion resistance and controlling thecoating weight, the coating weight is preferably not less than 10 g/m²or from a viewpoint of workability, the coating weight is preferably notmore than 120 g/m².

Following hot-dip galvanizing, a galvannealing process may be performedif necessary. The temperature at which the galvannealing process iscarried out is preferably in a range of 460˜550° C. At under 460° C.,the galvannealing reaction progresses slowly, but at 550° C. or higher,excess galvannealing occurs, and a hard and brittle Zn—Fe galvanneallayer is formed so much at the interface between the coating layer andthe matrix, thus not only deteriorating the coating adhesion, butdecomposing the retained austenite layer and aggravating thestrength-ductility balance. Meanwhile, when the Fe content in thecoating layer after the galvannealing process is under 7%, a soft Zn—Fegalvanneal layer is formed at the surface of the coating layer, therebyworsening the slidablity. On the other hand, the Fe content of over 15%is not preferable because the hard and brittle Fe—Zn galvanneal layer isformed at the interface of the matrix in the coating layer, lowering thecoating adhesion.

A high strength steel sheet according to this embodiment may not only behot-dip galvanized but also may be coated with hot-dip Zn-5% Al coating,hot-dip Zn-55% Al coating, hot-dip Al coating, or the like. The kind ofa steel sheet to which this embodiment is applied, whether cold-rolledor hot-rolled, does not matter so long as desired mechanical propertiescan be obtained.

EMBODIMENT 1

A steel slab of a chemical composition shown in Table 9 was heated at1150° C. for 25 minutes in a heating furnace, then hot-rolled to athickness of 2.8 mm, and coiled at 450˜780° C. to obtain a hot-rolledstrip. Subsequently, with the skin scale removed by acid cleaning, thestrip was cold-rolled to a thickness of 1.4 mm, and subjected to acontinuous annealing annealed at 800˜850° C. for recrystallization, andoveraged at 400˜500° C. to thereby finish a cold-rolled sheet.

This cold-rolled strip was further subjected to various kinds ofcoating, such as electro-galvanizing, electro Zn—Ni electroplating or Znphosphating, and the steel sheet was evaluated in external appearance,coating adhesion (only for electroplated sheets), and corrosionresistance (only for Zn-phosphated sheets). The electroplated coatingweight was 20 g/m² and the chemical treatment weight was 2 g/m².

TABLE 9 Steel Chemical Composition (mass %) No. C Si Mn P Al Cu Ni Mo NbRemarks 1 0.08  0.01 1.6 0.01 2.0 — — — — Inventive Steel 2 0.09 0.3 1.70.009 1.5 — — — — Inventive Steel 3 0.10 0.6 1.8 0.03 0.4 — — — —Inventive Steel 4 0.07 0.1 1.4 0.0012 1.3 0.2 0.1 0.1 — Inventive Steel5 0.11 0.2 1.9 0.0013 1.8 — — 0.05 0.03 Inventive Steel 6 0.06 1.6 1.90.02 0.2 — — — — Comparative Steel 7 0.075 0.8 1.8 0.011  0.01 — — — —Comparative Steel

The properties of steel sheets obtained as described are evaluated asfollows.

External appearance: Bare spots or the uneven adhesion were observedvisually, and specimens without flaws are decided as good (∘).

Coating adhesion: Electroplated steel sheets were subjected to a ballimpact test, and then a cellophane tape was attached to the processedparts and peeled off. Whether or not the coating layer was peeled offwas visually observed, and “without bare spots” was classified as ∘,“there are somewhat bare spots” was classified as Δ, and “bare spotsoccurred conspicuously” was classified as X. Corrosion resistance:Zn-phosphated steel sheets were electrodeposited, a cross was engravedwith a knife, and test specimens were dipped in a 5% NaCl or 55° C. saltwater for 240 hours, taken out and dried, and tape peeling test wasperformed on cross-cut portions, and the peeling width was measured.Those with a peeling width of less than 3.5 mm were classified as good(∘), those with a width of 3.5 mm˜under 4 mm were classified as rathergood (Δ), and those with a width of not less than 4 mm were classifiedas bad (X).

For cold-rolled steel sheets yet to be coated, the weldability wasevaluated as follows.

Weldability test: Two test specimens were spot-welded together by usinga dome-tipped 6-mm dia. welding electrode under conditions of electrodepressure of 4.3 kN, welding current of 8 kA, squeeze time for 25 cycles,setup time for 3 cycles, welding time for 13 cycles, and holding timefor 1 cycle; subsequently, a maximum tensile load in a tensile sheartest (TSS) according to JIS Z3136 and a maximum tensile load in a crossjoint tensile test (CTS) according to JIS Z3137 were measured. Thosetest specimens for which ductility ratio (CTS/TSS) was not less than0.25 and the tensile load (TSS) was not less than the standard tensileshear load (1162N) for a thickness of 1.4 mm were classified asexcellent (∘), and those not meeting the above conditions wereclassified as inferior (X).

On cold-rolled steel sheets before sent to coating, the amounts ofinternal oxides were measured by the above-mentioned method, and theinternal oxides were identified. If an oxide existed in a range up to0.1 μm from the surface, such a case was determined as “presence ofoxide”.

The test results are shown en bloc in Table 10. As is clear from thisTable, high tension steel sheets according to the present invention,regardless of large contents of Al and Si, invariably exhibit excellentsurface treatability, coatability, spot-weldability, and post-paintingcorrosion resistance.

TABLE 10 Hot- CAL Annl. CAL Dew Oxide In Surf. Amt. Of Oxide Ex. No.Steel No. Roll. CT Temp. (° C.) Point (° C.) Layer Al Oxide Other Oxides(G/m²) Embodied 1 1 450 800 −40 Present Present MnO 0.01 Ex. 2 1 450 800−40 Present Present MnO 0.01 3 1 450 800 −40 Present Present MnO 0.01 42 450 800 −30 Present Present SiO₂, MnSiO₃ 0.02 5 3 450 800 −30 PresentPresent SiO₂, MnSiO₃, 0.02 P₂O₅ 6 4 450 800 −30 Present Present SiO₂,MnSiO₃, 0.02 P₂O₅ 7 5 450 800 −30 Present Present SiO₂, MnSiO₃, 0.02P₂O₅ 8 1 780 800 −40 Present Present MnO 1.2 Comp. Ex. 1 2 450 800 −60Absent Absent Absent 0 2 3 450 800 −60 Absent Absent Absent 0 3 6 450800 −5 Present Present SiO₂, MnSiO₃, 0.3 Mn₂SiO₄, P₂O₅ 4 7 450 800 −30Present Absent SiO₂, MnSiO₃, 0.02 Mn₂SiO₄, P₂O₅ Corr. Resist. (Corr.External Coating Peel-Off Ex. No. Coating Appearance Adhesion Width)(mm) Weldability Embodied 1 Electro Galv. ◯ ◯ — ◯ Ex. 2 Zn—Ni E.P. ◯ ◯ —◯ 3 Zn phos. Ch. Treat. ◯ — ◯(2.1) ◯ 4 Zn phos. Ch. Treat. ◯ — ◯(2.3) ◯5 Zn phos. Ch. Treat. ◯ — ◯(1.6) ◯ 6 Zn phos. Ch. Treat. ◯ — ◯(1.8) ◯ 7Zn phos. Ch. Treat. ◯ — ◯(3.1) ◯ 8 Zn phos. Ch. Treat. ◯ — Δ(3.8) ◯Comp. Ex. 1 Zn phos. Ch. Treat. Uneven Adhesion — X(5.4) X 2 Zn phos.Ch. Treat. Uneven Adhesion — X(6.5) X 3 Zn phos. Ch. Treat. SlightlyUneven — X(6.4) ◯ Adhesion 4 Zn phos. Ch. Treat. ◯ — X(8.0) ◯

EMBODIMENT 2

A steel slab of chemical composition shown in Table 9 same as inEmbodiment 1 was heated at 1150° C. for 25 minutes in a heating furnace,then hot-rolled to a thickness of 2.8 mm, and coiled at 450˜780° C. toobtain a hot-rolled strip. After this, after its skin scale had beenremoved by acid cleaning, the strip was cold-rolled to a thickness of1.2 mm, and then annealed under conditions shown in Table 12 in a CGLline, and subsequently hot-dip galvanized and subjected to agalvannealing process at 450˜570° C. when necessary. The galvanizingbath temperature was held at 450˜460° C., and as for the galvanizingbath composition, three different compositions-were used: in addition toa Zn bath with Al content of 0.13˜0.20 mass %, a Zn bath with Al contentof 5 mass % and a Zn bath with Al content of 4 mass % and Mg content of1.5 mass % were used. The coating weight was adjusted to 50±5 g/m² foreach single side by gas wiping.

An obtained hot-dip galvanized steel sheet was analyzed to determine theamount of internal oxide and identify oxides as in Embodiment 1. Also,the external appearance, the degree of galvannealing (only ongalvannealed steel sheets), the coating adhesion and the corrosionresistance were examined.

Degree of galvannealing: The coating layer was dissolved in a mixedsolution of an aqueous solution of 20 wt % of NaOH and 10 wt % oftriethanolamine and an aqueous solution of 35 wt % of H₂O₂ mixed in avolume ratio of 195:7, and the solution is analyzed by ICP to measurethe Fe content (%).

External appearance evaluation: The external appearance was visuallychecked to find bare spots or unevenness of coating.

Coating Adhesion:

(Non-alloyed hot-dip galvanized steel sheet) After put to a ball impacttest, a cellophane tape was attached to a processed part of the steelsheet and the tape was peeled off. The test specimens were visuallyinspected to see if there is any peeled-off coating layer. Thosespecimens on which the coating did not peel off were classified as ∘,those specimens with the coating peeled off a little were classified asΔ, and those specimens on which the coating peeled off conspicuouslywere classified as X.

(Alloyed hot-dip galvanized steel sheet) A test specimen to which acellophane tape was attached was bent 90 degrees on the tape-attachedside and bent back, and the tape was peeled off; subsequently, theamount of the peeled-off coating per unit length was measured in termsof counts of Zn by a fluorescent X-ray. The number of counts of Zn wasevaluated referring to the criteria in Table 11. For measurement byfluorescent X-ray, an Rh bulb was used under condition of 40 kV and 50mA for 120 sec.

Corrosion resistance: After the surfaces of a steel sheet manufacturedby the above method were subjected to a chemical treatment process andan electrodeposition process, in test specimens was inscribed a crosswith a knife, and a test specimen was put to a CCT test for a total of50 cycles, one cycle consisting of a series of steps shown below, atape-peel-off test was conducted on the cross-inscribed portions and thepeeling-off width of the coating film was measured. A decision was givenas follows: a case where the peeling-off width was under 4 mm wasclassified as good (∘), a case where the width was 4 mm or larger wasclassified as defective (∘).

One cycle consists of wetting (2 hrs)—salt water spray (2 hrs)—drying (1hr)—wetting (6 hrs)—drying (2 hrs)—wetting (6 hrs)—drying (2 hrs)—lowtemperature (3 hrs).

A cold-rolled steel sheet yet to be galvanized was evaluated in terms ofweldability as follows.

Two test specimens were spot-welded together by using a dome-tipped6-mm-dia. Welding electrode under conditions of electrode pressure of3.1 kN, welding current of 7 kA, squeeze time for 25 cycles, setup timefor 3 cycles, welding time for 13 cycles, holding time for 1 cycle;subsequently, a maximum tensile load in a tensile shear test (TSS)according to JIS Z3136 and a maximum tensile load in a cross jointtensile test (CTS) according to JIS Z3137 were measured. Those testspecimens for which the ductility ratio (CTS/TSS) was not less than 0.25and the tensile load (TSS) was not less than the standard tensile shearload (8787N) for a thickness of 1.2 mm were classified as excellent (∘),and those not meeting the above conditions were classified as inferior(X).

TABLE 11 Rank Fluores. X-Ray Count Evaluation 1 0~500 Good 2 Over500~1000 Good 3 Over 1000~2000 Defective 4 Over 2000~3000 Defective 5Over 3000 Defective

The test results are shown en bloc in Table 12. As is clear from thisTable, hot-dip galvanized steel sheets according to the presentinvention, regardless of no small contents of Al and Si, all exhibitexcellent coating adhesion, spot-weldability, and post-paintingcorrosion resistance.

TABLE 12 Galvan- Steel Hot- CAL Annl. CAL Dew nealing Oxide In Surf. AlAmt. Of Ex. No. No. Roll. CT Temp. (° C.) Point (° C.) Temp. (° C.)Layer Oxide Other Oxides Oxide (G/m²) Embodied 9 1 450 800 −40 500Present Present MnO 0.01 Ex. 10 1 450 800 −45 500 Present Present Absent0.006 11 1 450 800 −10 500 Present Present MnO 0.1 12 1 450 800 −40 —Present Present MnO 0.01 13 1 450 800 −40 — Present Present MnO 0.01 142 450 800 −30 510 Present Present SiO₂, MnSiO₃, 0.02 15 3 450 800 −30530 Present Present SiO₂, MnSiO₃, 0.02 P₂O₅ 16 4 450 800 −30 500 PresentPresent SiO₂, MnSiO₃, 0.02 P₂O₅ 17 5 450 800 −30 500 Present PresentSiO₂, MnSiO₃, 0.02 P₂O₅ 18 1 780 800 −40 470 Present Present MnO 1.2Comp. Ex. 5 1 450 800 −60 500 Absent Absent Absent 0 6 2 450 800 −60 500Absent Absent Absent 0 7 3 450 800 −5 510 Absent Absent Absent 0 8 6 450800 −30 520 Present Present SiO₂, MnSiO₃, 0.3 Mn₂SiO₄, P₂O₅ 9 7 450 800−30 520 Present Absent SiO₂, MnSiO₃, 0.02 Mn₂SiO₄, P₂O₅ Degree OfGalvannealing External Coating Corr. Resist. (Corr. Ex. No. Kind OfCoating (Fe %) Appearance Adhesion Peel-Off Width) (mm) WeldabilityEmbodied 9 Alloyed Hot-Dip Galv. 12 ◯ ◯ ◯(0.8) ◯ Ex. 10 Alloyed Hot-DipGalv. 10 ◯ Δ ◯(0.86) ◯ 11 Hot-Dip Galv. — ◯ ◯ ◯(0.9) ◯ 12 Hot-Dip Zn-5%Al — ◯ ◯ ◯(2.1) ◯ Coating 13 Alloyed Hot-Dip Galv. — ◯ ◯ ◯(2.1) ◯ 14Alloyed Hot-Dip Galv. 11 ◯ ◯ ◯(0.75) ◯ 15 Alloyed Hot-Dip Galv.  9 ◯ ◯◯(1.4) ◯ 16 Alloyed Hot-Dip Galv. 10 ◯ ◯ ◯(0.76) ◯ 17 Alloyed Hot-DipGalv. 12 ◯ ◯ ◯(0.7) ◯ 18 Alloyed Hot-Dip Galv. 10 Uneven And Rough Δ Δ(3.7) ◯ Comp.Ex. 5 Alloyed Hot-Dip Galv. 10 Bare X X(4.8) X 6 AlloyedHot-Dip Galv. 11 Bare X X(5.1) X 7 Alloyed Hot-Dip Galv.  8 Bare XX(4.1) X 8 Alloyed Hot-Dip Galv.  8 ◯ ◯ X(4.2) ◯ 9 Alloyed Hot-Dip Galv.13 ◯ ◯ X(4.3) ◯

INDUSTRIAL APPLICABILITY

In the automotive industry, for example, the use of high-tensile steelsheets is increasing rapidly with a view to reducing car weight forbetter fuel economy and improved crashworthiness. The high-tensile steelsheet has a steel composition including addition elements such as Si,Mn, Ti, Al and P. It is well known, however, that as the Si content isincreased, an Si oxide film is formed on the surfaces of the steel sheetduring annealing, thus deteriorating the steel properties, such aschemical properties, electro-galvanized coating adhesion, hot-dipgalvanizability and coating adhesion. Above all else, the big problem isthe inferior hot-dip galvanizability of Si-containing high-tensile steelsheets. To be more specific, when a steel sheet is hot-dip galvanized,its poor wettability causes the applied zinc not to adhere to some partsof the steel sheet, which are the so-called “bare spots”, or aninsufficient adhesion occurs in which the coating separates during pressworking. As a method for achieving high ductility and high tensilestrength without increasing the Si content, there is a technique bywhich to positively increase the Al content in the steel to therebydecrease the Si content, the surface quality deterioration peculiar tothe Si-added steel can be prevented and simultaneously the retainedaustenite can be made stable.

However, Al and Si being both readily oxidizable elements, an Al oxidefilm is also formed in addition to the Si oxide film during annealing,and therefore the problems of deterioration in hot-dip galvanizabilityand coating adhesion have not been solved.

According to a coated steel sheet and a method for manufacturing thissteel sheet in the present invention, the diffusion of Al into thesurface layer of the steel sheet is prevented, the amount of Al solidsolution in the surface layer is decreased, and desired steel structureand mechanical properties can be secured. Moreover, the surfacetreatability, hot-dip galvanizability, post-painting corrosionresistance, and weldability can be improved. Furthermore, even if thesteel sheet has a high Al content, a coating can be formed withexcellent adhesion properties.

1. A coated steel sheet having a coated layer on a surface of a steelsheet of a composition containing not less than 0.1 mass % and under 3mass % of Al, wherein a following condition A or B is met: A: An AlNprecipitate layer exists on a matrix side near an interface between saidsteel sheet and said coated layer B: Oxide of Al exists in said matrixright under said surface of said steel sheet.
 2. A coated steel sheetaccording to claim 1, wherein said coated layer is a hot-dip galvanizedlayer containing 0.1˜1.0 mass % of Al.
 3. A coated steel sheet accordingto claim 2, wherein said coated layer is a Zn—Fe galvanneal coating thatfurther contains 7˜15 mass % of Fe.
 4. A coated steel sheet according toclaim 2, wherein there is an AlN precipitate layer on the matrix sidenear an interface between said steel sheet and said coated layer andsaid AlN precipitate layer has a thickness of not less than 1 μm and notmore than 100 μm.
 5. A coated steel sheet according to claim 2, whereinsaid steel composition further contains one or two kinds selected fromnot less than 0.1 mass % of Si and not less than 0.5 mass % of Mn.
 6. Acoated steel sheet according to claim 2, wherein said steel compositionfurther contains one or two kinds selected from not less than 0.01 mass% and not more than 1 mass % of Mo and not less than 0.005 mass % andnot more than 0.2 mass % of Nb.
 7. A coated steel sheet according toclaim 2, wherein said steel composition further contains not less than0.01 mass % and not more than 0.5 mass % of Cu, not less than 0.01 mass% and not more than 1 mass % of Ni, and not less than 0.01 mass % andnot more than 1 mass % of Mo.
 8. A coated steel sheet according to claim2, wherein said steel composition further contains 0.03˜0.25 mass % ofC, 0.001˜1.0 mass % of Si, 0.5˜3.0 mass % of Mn, and 0.001˜0.10 mass %of P.
 9. A coated steel sheet according to claim 8, wherein in saidsteel matrix, there are one or more kinds selected from oxides of SiO₂,MnO, FeSiO₃, Fe₂SiO₄, MnSiO₃, Mn₂SiO₄, and P₂O₅.
 10. A coated steelsheet according to claim 8, wherein the amount of oxides in total perone side surface is 0.01˜1.0 g/m².
 11. A coated steel sheet accordingclaim 8, wherein the steel composition contains 0.01˜1.0 mass % of Moand 0.005˜0.2 mass % of Nb.
 12. A coated steel sheet according to claim8, wherein the steel composition is 0.01˜0.5 mass % of Cu, 0.01˜1.0 mass% of Ni, and 0.01˜1.0 mass % of Mo.
 13. A coated steel sheet accordingto claim 8, wherein the coated layer is a hot-dip galvanized layer andis galvannealed.
 14. A coated steel sheet according to claim 8, whereinthe Fe content in the galvannealed coated layer is 7˜15 mass %.
 15. Amethod for manufacturing a coated steel sheet comprising the steps ofheating and holding a steel slab and hot-rolling said slab, andhot-dip-galvanizing a steel sheet, wherein said slab contains not lessthan 0.1 mass % under 3 mass % of Al, and said holding is carried out inan atmosphere containing not less than 1 vol % and not more than 20 vol% of O₂ and not less than 70 vol % of N₂ under the conditions that meetan equation (1) shown below and said galvanizing is performed by using agalvanizing bath with an Al concentration in the bath is 0.14˜0.24 mass% at a bath temperature of 440˜500° C.{Heating and holding temp.(° C.)−(1050+25Al)}×heating and holding time(min)≧3000  (1) wherein Al denotes an Al content (mass %) in the steel.16. A method for manufacturing a coated steel sheet according to claim15, wherein said steel sheet is galvanized by using a galvanizing bathof Al concentration of 0.10˜0.20 mass % in the bath at a bathtemperature of 440˜500° C. and the hot-dip-galvanized layer is furthersubjected to a galvannealing process at 460˜550° C.
 17. A method formanufacturing a coated steel sheet according to claim 15, whereincold-rolling is carried out between the hot-rolling process and thehot-dip galvanizing process.
 18. A method for manufacturing a coatedsteel sheet according to claim 15, wherein said steel slab furthercontains one or two kinds selected from not less than 0.1 mass % of Siand not less than 0.5 mass % of Mn.
 19. A method for manufacturing acoated steel sheet according to claim 15, wherein said slab furthercontains one or two kinds selected from not less than 0.01 mass % andnot more than 1 mass % of Mo and not less than 0.005 mass % and not morethan 0.2 mass % of Nb.
 20. A method for manufacturing a coated steelsheet according to claim 15, wherein said slab further contains not lessthan 0.01 mass % and not more than 0.5 mass % of Cu and not less than0.01 mass % and not more than 1 mass % of Ni, and not less than 0.01mass % and not more than 1 mass % of Mo.